1. Field of Invention
This invention is directed to apparatus and methods for providing consistent output color across a plurality of different hard copy output devices.
2. Description of Related Art
When a user creates a document or image having, for example, a plurality of colors, the colors each have values associated with them in order to categorize the color in such a way that the color may be reproduced. For example, with a cathode ray tube (CRT) device, computer monitor, and other such similar display devices, the colors of individual pixels are identified in terms of the red (R), green (G) and blue (B) components of the color to be displayed for the pixel. For devices that provide hard copy images, such as for example, printers, photocopiers, and the like, the colors that are output are identified by their cyan (C), magenta (M), yellow (Y) and black (K) components.
When an image displayed on a display device, such as a CRT or computer monitor, is to be converted into a hard copy of the image, the RGB values of the display must be converted into the CMYK values that are useable by the hard copy output device. This is typically done by converting the RGB into CMYK values using a look-up table. The look-up table includes values that approximate a transformation from RGB to a device-independent color space representation, such as L* a* b* which represents the colors perceived by the human eye, and then from device-independent color space representation into CMYK values.
Each hard copy output device has a limited gamut of colors that can be reproduced by the device. Because of these gamut limitations, not all of the RGB colors provided by the display device may be accurately represented on a hard copy. Typically, when the desired RGB colors lay outside the gamut of the particular hard copy output device, the desired RGB colors are mapped to colors that can be output by the hard copy device. Thus, there is a difference in the colors seen by the human eye and the colors that are output by the hard copy output device and hence, the desired colors of the image cannot be obtained using the hard copy output device.
Similarly, each hard copy output device may have its own characteristics that cause it to output a different color from colors of other hard copy output devices. For example, the output colors may be dependent on the fact that the hard copy output device uses a different type of color ink or toner, has a low ink or toner state, and the like. Additionally, over time, the output of the hard copy output device may drift, i.e. deviate from predetermined optimum standards, due to various factors. These factors include environmental conditions, such as temperature, relative humidity, and the like, use patterns, the type of media used, such as different paper types, transparencies, and the like, variations in media, variations from original models used in initializing the hard copy output device, general wear, and the like. Thus, even though the color specified by the RGB values may be within the gamut of the hard copy output device, the desired color may still not be achieved due to a current status of the hard copy output device.
This invention provides apparatus and methods that obtain consistent results across a plurality of different hard copy output devices.
This invention separately provides apparatus and methods that provide consistent output image quality across a plurality of different hard copy devices by adjusting input color values based on detected color values.
One exemplary embodiment of a system according to this invention comprises an image data source, a printable image data adjusting system, a hard copy output device and an image sensor. The image data source supplies image data to the printable image data adjusting system. The image data may be in a device-dependent color space or a device-independent color space. If the image data is in a device-dependent color space, the printable image data adjusting system converts the image data into device-independent image data and stores the device-independent image data in a memory as target image data. If the image data is already device-independent image data, the device-independent image data is simply stored in the memory of the printable image data adjusting system as the target image data.
The printable image data adjusting system then uses the target image data to generate printable image data that is in a color space useable by the hard copy output device, such as CMYK image data. The printable image data adjusting system then supplies this printable image data to the hard copy output device, which generates a hard copy of the image. The image is then placed within the optical field of the image sensor. The image sensor detects color values of device-independent image data from the image generated by the hard copy output device. The detected color values of device-independent image data is then provided to the printable image data adjusting system, which compares the detected color values of device-independent image data to the color values of the target image data to generate color adjustment factors.
The color adjustment factors are then applied to generate adjusted printable image data. This color adjusted printable image data is used by the hard copy output device to generate a new hard copy image. This new hard copy image should result in, for example, detected device-independent image data that more closely represents the target image data. Alternatively, the printable image data may be adjusted to obtain any desired effect. The process may be iteratively performed until a particular tolerance is obtained or a maximum number of iterations is achieved. The process may also be performed in real time.